Dosing unit and method for dosing liquid or gaseous educts for a fuel cell system

ABSTRACT

The invention relates to a metering unit for metering liquid and/or gaseous educts by means of a feed pump for a fuel cell system. The metering unit includes at least one lead line ( 16 ) for delivering an educt flow, at least one control piston ( 12 ), and a differential pressure valve ( 14 ) for regulating the educt flow; the differential pressure valve ( 14 ) has a regulatable throttling cross section ( 39 ), which is variable automatically, as a function of a flow pressure dictated by the control piston ( 12 ), in order to regulate the educt flow. The lead line ( 16 ) leads to the control piston ( 12 ), and between the control piston ( 12 ) and the differential pressure valve ( 14 ) a first and a second connecting line ( 28, 29 ) are provided.

[0001] The invention relates to a metering unit and a method formetering liquid or gaseous educts for a fuel cell system, as genericallydefined by the preambles to the independent claims.

PRIOR ART

[0002] Among the alternative drive concepts for motor vehicles, fuelcell-supported systems are gaining increased attention at present. Thesesystems typically contain PEM (polymer electrolyte membrane) fuel cells,which are operated with hydrogen and air as energy vehicles.

[0003] Since as before, it still proves problematic to put hydrogen in atank and store it in the motor vehicle, the hydrogen is produced in anupstream reformer stage, from easily handled fuels such as methanol,methane, Diesel or gasoline, directly “onboard” as needed and isconsumed immediately. In such fuel cell systems, many flows of materialmust accordingly be metered flexibly and nevertheless quite precisely.This is true both for liquid components, such as superpure water, fuelsand coolant, and for such gaseous media as air or methane. The primaryproblem in the metering is that pressure fluctuations in the transportlines make exact metering of the individual components more difficult.These pressure fluctuations can be caused on the one hand by upstreampumps or compressors, but also by the chemical reactions that take placein the reformer, for instance, and can release process gases and thuslead to pressure return shocks.

[0004] In German Patent DE 44 25 634 C1, a method and an apparatus formetering liquids for a fuel cell system are described; the metering isdone via the cycle time of a magnet valve, and the pressure differencebetween the feed line and the fuel cell system is regulated via adifferential pressure valve. This arrangement, because clocked switchingvalves are used, creates fluctuations in the volumetric flow in thetransport lines, which can cause problems in the chemical reactions thattake place in the reactors of the fuel cell system.

[0005] In motor vehicles, fuel injection systems are used, which make itpossible to meter the fuel at different pressure conditions. Thesesystems, known by the trademark K-Jetronic, contain a combination of acontrol piston and a differential pressure valve. These areair-pressure-controlled systems, whose use is limited to the metering offuels.

[0006] The object of the present invention is to furnish a metering unitfor liquid and gaseous components for a fuel cell system. Pressurefluctuations inside the transport lines are compensated for, and exactmetering is made possible.

ADVANTAGES OF THE INVENTION

[0007] The metering unit of the invention and the method having thedefinitive characteristics of independent claims have the advantage thateven under dynamic load changes, exact metering of liquid and gaseousmedia is made possible, without requiring a complicated measurement ofvolumetric flows and corresponding regulation. Since the metering unitof the invention, in contrast to clocked systems, realizes a continuousmetering concept, pressure fluctuations in the transport lines of thefuel cell system are successfully averted. This is achieved by thecombination of a control piston with a differential pressure valve.Another advantage is that no parts that move at high speed are exposedto such corrosive media as superpure water, and the result is a markedlylonger service life for the metering unit.

[0008] By the provisions recited in the dependent claims, advantageousrefinements of and improvements to the metering unit and the methodrecited in the independent claims are possible.

[0009] For instance, for the most precise possible metering, it isadvantageous if the position of the control piston of the metering unitis determined by means of a travel sensor and can be varied by means ofa proportional magnet.

[0010] Furthermore, an adaptation of the metering unit to the particularliquid or gaseous media is possible, since in the connecting linesbetween the control piston and the differential pressure valve of themetering unit, there is a throttle, by which the pressure drop at themetering unit can be adapted.

[0011] To make extremely dynamic metering possible, in a furtheradvantageous feature, a drain line with a throttle is provided at thelower valve chamber of the differential pressure valve. This drain linepermits a rapid change in the control pressure applied to thedifferential pressure valve.

[0012] It is especially advantageous that for each medium, only one feedpump for furnishing pressure is required, and the metering can be doneby means of the metering unit of the invention. This makes itunnecessary to use expensive metering pumps.

DRAWINGS

[0013] One exemplary embodiment of the invention is shown in the drawingand explained in further detail in the ensuing description. FIG. 1 showsa schematic illustration of a first exemplary embodiment of the meteringunit of the invention; FIG. 2 shows a schematic illustration of themetering unit of the invention in a second exemplary embodiment; andFIG. 3 shows a schematic illustration of a fuel cell system, using themetering unit of the invention.

EXEMPLARY EMBODIMENTS

[0014] The metering unit 10 shown in FIG. 1 includes a control piston 12and a differential pressure valve 14. Upstream from the metering unit 10is a feed pump 11, for pumping the liquid or gaseous media required in afuel cell system. A system pressure regulator 13, for instance, can beconnected parallel to the feed pump 11 and regulates the system pilotpressure in the lead line 16 that connects the feed pump 11 to themetering unit 10. Inside the metering unit 10, the lead line 16 has acylindrical piston portion 16 a, in which the control piston 12 isguided adjustably, at least in part, by its end 12 a toward the leadline. The cylindrical piston portion 16 a for instance has a largercross section than the lead line 16. The control piston 12 has its end12 b toward the housing located outside the cylindrical piston portion16 a. Located on the housing end 12 b of the control piston 12 is aproportional magnet 18, for instance, for varying the position of thecontrol piston 12 inside the cylindrical piston portion 16 a.

[0015] For the sake of the best possible detection of the position ofthe control piston 12 inside the cylindrical piston portion 16 a of thelead line 16, a travel sensor 20 is also disposed on the housing end 12b of the control piston 12.

[0016] On its end 12 a toward the lead line, the control piston 12 has acontrol edge 22, whose position, together with the wall of thecylindrical piston portion 16 a, dictates a first throttling crosssection 24. In the region of the cylindrical piston portion 16 apreceding the lead line end 12 a of the control piston 12, there is acompression or tension spring 26, for instance, and a vent 27.

[0017] The cylindrical piston portion 16 a communicates with thedifferential pressure valve 14 by means of two connecting lines 28, 29.The differential pressure valve 24 in turn has an upper valve chamber 34and a lower valve chamber 32. The two valve chambers 32, 34 areseparated from one another by a flexible diaphragm 36. However, theseparation can also be effected by means of a movable, spring-supportedpiston.

[0018] The lower connecting line 28, which can for instance have athrottle, not shown, discharges into the lower valve chamber 32. Via theconnecting line 28, the lower valve chamber 32 is subjected to thesystem pilot pressure, which is generated by the feed pump 11 and iscorrected by the system pressure regulator 13.

[0019] The upper connecting line 29 discharges into the upper valvechamber 34 of the differential pressure valve 14. The flow pressureprevailing in the upper connecting line 29 and thus also in the uppervalve chamber 34 is predetermined by the position of the control edge 22and the thus-dictated throttling cross section 24.

[0020] The diaphragm 36 is connected, for instance by means of a tensionor compression spring 38, to the housing of the differential pressurevalve 14, so that the diaphragm 36 can react quickly and reversibly tochanges in the pressure conditions.

[0021] The upper valve chamber 34 also has a drain line 40, by way ofwhich the liquid and gaseous media carried into the upper valve chamber34 can be drained out in a metered quantity. The drain line 40 protrudesinto the upper valve chamber 34 far enough that a second throttlingcross section 39 is created between the inlet-side opening of the drainline 40 and the diaphragm 36. The size of this throttling cross sectionis automatically regulated in accordance with the magnitude of thesystem pilot pressure and of the flow pressure, the latter beingdictated by the first throttling cross section 22. Pressure fluctuationsin the lead line 16 and the drain line 40 are likewise automaticallycompensated for.

[0022] In FIG. 2, a second exemplary embodiment of the metering unit ofthe invention is shown. It includes a differential pressure valve 14′,whose lower valve chamber 32′ has a drain line 42, which includes avariable throttle 44 and by way of which the particular medium to bemetered is returned to a supply tank. The lower connecting line 28′ hasa fixed throttle 43, by way of which the system pilot pressure isreduced. The drain line 42 permits varying the system pilot pressureapplied to the lower valve chamber 32′ and in this way makes it possibleto vary the throttling cross section 39 of the differential pressurevalve. This is necessary above all for the sake of fast adaptations ofthe volumetric flow, passing through the metering unit 10, to thedynamic load changes that occur in the fuel cell system, in the eventthat control by way of the position of the control piston is toosluggish. If a measurement of the volumetric flow is also performedinside the metering unit 10, then with the aid of this correctivedevice, the metering precision of the metering unit 10 can be increasedconsiderably.

[0023] At first glance, combining a control piston 12 with adifferential pressure valve 14 may appear complicated compared to asimple throttle device, but it offers major advantages. The system pilotpressure, generated by the feed pump 11 and corrected by the systempressure regulator 13, generally drops not only at a throttle deviceused for metering purposes but also at throttling components inside theline system. A linear change in the throttling cross section of athrottle device hence causes a nonlinear change in the flow pressure inthe line system. Coupling two throttle devices (the control piston 12and the differential pressure valve 14, 14′), whose throttling crosssections 24, 39 dictate one another, to make a metering unit 10 leads toa constant pressure drop at the metering unit 10, given a constantposition of the control piston 12, and thus to a proportionality of thepressure drop and the first throttling cross section 24. Given asuitable design of the control edge 22, a proportionality is furthermoreobtained between the piston stroke of the control piston 12 and thevolumetric flow passing through the metering unit 10.

[0024] In FIG. 3, a fuel cell system 50 is schematically shown, in whichthe use of the metering unit of the invention will be explained as anexample.

[0025] The generation of the hydrogen required for the fuel celloperation takes place directly in the fuel cell system 50, in aso-called reformer 51. The hydrogen is obtained by partial oxidation offuels with the addition selectively of water vapor, air, or a mixture ofthe two. The reaction typically takes place in a heatable catalyticconverter; as the fuels, gasoline, Diesel, methane or methanol can beused. Methanol and water mixtures, or emulsions of gasoline and water,are also suitable. All the educts are delivered in gaseous form to thereformer 51. A prerequisite is an evaporator for a fuel 53 andoptionally also for water 52. The requisite energy can be furnished viaa catalytic burner 54, for instance.

[0026] The gas flow leaving the reformer contains major quantities ofCO, which would inactivate the catalysts contained in PEM fuel cells.For this reason, a plurality of chemical cleaning stages 55, 56 areintegrated into the system between the reformer 51 and the fuel cells62; with the addition of water, these stages convert the carbon monoxideinto carbon dioxide and hydrogen. In addition, optional heat exchangers57, 58 are provided downstream of the cleaning stages, in order todissipate the reaction heat.

[0027] A metering of fuel by means of the metering unit of the inventionis preferably effected in such a system between a fuel tank 59 and theevaporator 52 at a first point 101, or between the fuel tank 59 and thereformer 51 at a second point 102, and as needed between the fuel tank59 and the catalytic burner 54 at a third point 103.

[0028] Provision is made for metering superpure water between a watertank 60 and the heat exchangers 57, 58 at a fourth point 104, betweenthe water tank 60 and the fuel cells 62 at a fifth point 105, betweenthe water tank 60 and the cleaning stages 55, 56 at a sixth point 106,and between the cleaning stage 55 and an evaporator 53 at a seventhpoint 107.

[0029] Depending on the method variant, an admixture of metered air mayalso be needed. This is done above all between a compressor 61 and thecatalytic burner 54 at an eighth point 108, between the compressor 61and the cleaning stage 56 at a ninth point 109, between the compressor61 and the fuel cells 62 at a tenth point 110, and between thecompressor 61 and a reformer 51 at a further point 111.

[0030] The metering unit of the invention is not limited to theexemplary embodiments described; on the contrary, further features of ametering unit with two coupled throttle devices are also conceivable.Moreover, the metering unit of the invention can be coupled with anatomizer, so that liquid educts can for instance be delivered in ametered quantity and in superfinely distributed form to the reformer.

1. A metering unit for metering liquid and/or gaseous educts by means ofa feed pump for a fuel cell system having at least one lead line fordelivering an educt flow, at least one control piston, and adifferential pressure valve for regulating the educt flow, wherein thedifferential pressure valve has a regulatable throttling cross sectionwhich can be varied automatically, as a function of a flow pressuredictated by the control piston, in order to regulate the educt flow,characterized in that the lead line (16) leads to the control piston(12), and from the control piston (12) a first and a second connectingline (28, 28′, 29) lead to the differential pressure valve (14, 14′). 2.The metering unit of claim 1, characterized in that the position of thecontrol piston (12) in the lead line (16) is variable by means of aproportional magnet (18).
 3. The metering unit of claim 1 or 2,characterized in that the control piston (12) has a control edge (22)whose position in the lead line (16) predetermines the size of a furtherregulatable throttling cross section (24) for regulating the educt flow.4. The metering unit of at least one of claims 1-3, characterized inthat a travel sensor (20) is provided, for determining the position ofthe control piston (12) in the lead line (16).
 5. The metering unit ofat least one of claims 1-4, characterized in that the differentialpressure valve (14, 14′) includes a movable, spring-supported piston,whose position predetermines the throttling cross section (39) of thedifferential pressure valve (14, 14′), for regulating the educt flow. 6.The metering unit of at least one of claims 1-4, characterized in thatthe differential pressure valve (14) includes a movable diaphragm (36),whose deformation predetermines the throttling cross section (39) of thedifferential pressure valve (14, 14′), for regulating the educt flow. 7.The metering unit of claims 5 and 6, characterized in that thedifferential pressure valve (14, 14′) includes an upper valve chamber(34) and a lower valve chamber (32, 32′), which are separated from oneanother by the piston or the diaphragm (36).
 8. The metering unit ofclaim 7, characterized in that the lower valve chamber (32, 32′) of thedifferential pressure valve (14, 14′) is subjected to the liquid or gaspressure prevailing in the lead line (16), and the upper valve chamber(34) is subjected to the liquid or gas pressure that is reduced by thecontrol piston (12).
 9. The metering unit of at least one of claims 1-8,characterized in that at least one of the connecting lines (28, 28′, 29)has a throttle (43).
 10. The metering unit of at least one of claims7-9, characterized in that the lower valve chamber (32′) has a drainline (42), and that the drain line (42) contains a further throttle(44).
 11. A method for metering liquid or gaseous educts by means of ametering unit for a fuel cell system of at least one of claims 1-10,characterized in that to avoid pressure fluctuations, a differentialpressure valve (14, 14′) is used, whose throttling cross section (39)varies automatically as a function of a flow pressure dictated by acontrol piston (12).
 12. The method of claim 11, characterized in thatby means of the metering unit, metering of fuel is effected between afuel tank (59) and an evaporator (52) at a first point (101) and/orbetween the fuel tank (59) and a reformer (51) at a second point (102),and/or between the fuel tank (59) and a catalytic burner (54) at a thirdpoint (103).
 13. The method of claim 11 or 12, characterized in that bymeans of the metering unit, metering of water is effected between thewater tank (60) and a heat exchanger (57, 58) at a fourth point (104),and/or between the water tank (60) and a fuel cell (62) at a fifth point(105), and/or between a water tank (60) and a cleaning stage (55, 56) ata sixth point (106), and/or between the cleaning stage (55) and anevaporator (53) at a seventh point (107).
 14. The method of at least oneof claims 11-13, characterized in that by means of the metering unit, ametering of air is effected between a compressor (61) and a catalyticburner (54) at an eighth point (108), and/or between the compressor (61)and a cleaning stage (56) at a ninth point (109), and/or between thecompressor (61) and a fuel cell (62) at a tenth point (110), and/orbetween the compressor (61) and a reformer (51) at a further point(111).
 15. The use of a metering unit of at least one of claims 1-10 foratomizing liquid educts of a reformer for fuel cells.